Two-way amplifier for eliminating oppositely directed signals



du. R N5@ G. E. @mamma ETAL TWO-WAY AMPLIFIER FOR ELIMINATING OPPOSTELYDIRECTED SIGNALS Filed Nov. are, 1947 m' i? ff A TTORNE'Y Patented Feb.7, i956 TWO-WAY AMPLIFIER FOR ELIMINATING l OPPOSITELY DIRECTED SIGNALSGeorge E. Brodo. Newark, and Hayes B. Steinhauser, Montclair, N. J.,assignors to Federal Telephone and Radio Corporation, New York, N. Y., acorporation of Delaware 'Application November 1s, 1947,'seria1 No.786,728

6 Claims. (Cl. 179-170) Our invention relates to two-way amplifier foreliminating oppositely directed signals and is particularly directed toamplifiers for relaying voice signals traveling in either direction in atwo wire transmission circuit.

The two-way ampliers standardized for ordinary two-Wire telephoneservice are more complex and more expensive than can be justified inmany smal] installations such as sound-powered systems. In airportcontrol systems, for example, the noise level at certain stations mayrequire amplifiers in some of 'the interconnecting telephone lines. Alarge item of cost in all of twoway amplifiers for two-wire, or singlecircuit, systems is in the circuits for preventing feedback andoscillations at the ampliiiers, the 'hybrid coil and balanced bridgebeing the central features of conventional means for isolatingoppositely directed signal currents.

The general objects of our invention is improved two-way amplifiers.

A more specic obiect of our invention is simplified means for isolatingoppositely directed signal currents in two-way ampliers.

Other and more specific objects of our invention will appear in thefollowing specification in which a preferred embodiment of our inventionis described. The appended claims deline the scope of our invention andthe accompanying drawing shows said preferred embodiment.

In the drawing:

Fig. 1 is a diagram of the signal circuits only of the preferredembodiment of our two-way ampliiier; and

Fig. 2 is a circuit diagram of a commercial embodiment of the amplifierof Fig. 1.

The two-wire signal circuit, in which a twoway amplifier is to beinserted, is for convenience divided, the two portions I and 2 beingidentified as west and east lines, respectively. Because the two-wirecircuits must conduct signal voltages in both directions, the input andoutput circuits of the amplifiers must be cou.- pled together, and yetthe amplifiers must not sing or oscillate. Two four-sided Wheatstonebridge networks 3 and 4 are shown, impedances 5 and 6 in bridge 3,preferably being resistive in nature and in adjacent arms of the bridge.The other two arms of bridge 3 comprise the anodecathode resistances ofamplier tubes 'i and 8, the cathodes being connected together and to acommon cathode resistor. Control grids 9 and I il are important, butscreen or suppressor grids are optional, in the operation to bedescribed. The tubes II and I2, with grids I3 and It, and im- 2 pedancesI5 and I6 comprise the four arms of bridge 4.

The west line is coupled across one diagonal a--b of bridge 3 by way ofthe coupling transformer with primary and secondary windings I'I and I8,respectively, the transformer being shown because it is one of the moreconvenient impedance matching devices. In series with the transformersecondary is an additional impedance I9 having the approximate ohmicvalue of the connected transformer winding. The junction of elements I8and I9, which is the electrical center of the diagonal when I8 and YI9are equal is grounded. Thus, because of the asymmetry of the transformerI8 in the diagonal, signals applied to the one diagonal a-b, from line Iwill appear across the other diagonal c-d, or between apex c and ground.

The voltage at c of bridge 3 is applied to the grid III of amplifier I2of bridge 4, the corresponding apex of bridge l being connected to thegrid Ill of `amplier 3 of bridge 3. The grids 9 and I3 of the remainingamplifier tubes are directly grounded, and the cathodes of theamplifiers of either bridge are connected togetherand to ground througha common cathode resistor, the cathode resistors for the two pairs oftubes being shown at 20 andZI.

Assume a signalarrives over line I. This signal will appear at apex cand hence at the grid I4 of amplilier I2. Now, since the space currentof amplifier I2 flows through cathode resistor 2I, the signal Voltagewill appear across resistor 2|. But since resistor ZI is directly acrossthe input of amplier tube I I, the signal voltage is received andampliiied by tube II, and since the on-e tube receives its signal fromthe cathode resistor of the other tube, the signal voltage variations atthe two anodes are in phase opposition. (Load resistors and anode powersupplies are purposely omitted in Fig. 1.) The effective push-pulloperation of the amplifiers II and I2 accordingly apply at apices and yof bridge 4 an amplified signal which, owing in the :c--y diagonalcircuit, is applied to the "east line 2. But the signal from westappearing at x--y will produce no signal voltage across the otherdiagonal w-z with impedances I 5 and I t balanced, and will transmit nopart of the west signal from z to grid IIJ. oscillations and singing dueto feedback are effectively prevented. Signals arriving from the eastline are likewise amplified and fed into the west line withoutoscillations.

One commercial embodiment of our novel two- Way amplifier is shown inFig. 2, the reference characters in the two figures being the same forlike parts, Load resistors 22 and 23 are connected to the anodes ofampliers 'i and 8, respectively, and at their common junction areconnected to a high anode-voltage source. Load resistors 2A and 25 aresimilarly connected to the anodes of ampliers II and I2. To obtain equaloutput voltages from amplifiers I3 and I4, the plate load resistor 2should be smaller than resistor 25 because of the difference inamplitude of the signal applied to the cascaded grid circuits. The usualblocking condensers 26, 21, 28 and 29 keep direct current and highvoltages out of the signal circuits. The reactance oi condenser 3i] maybe adjusted to balance the re actance of winding I8.

In one system similar to that of Fig. 2, ampliiiers 1 and 8, as well asI3 and I4, comprised the triode sections of a commercial 6SN'7. VTheanode voltages Were 250 Volts, and the value of resistors 5, 0, I5, I6was about 100,000 ohms each. Initial balance was made by applying asignal at the east line and adjusting resistor 5 until oscillationscease. Resistor I could be adjusted with the saine result. Then a signalwas applied at the west line and resistor I adjusted until oscillationsceased.

The circuit components of our two-Way amplifier may be comprisedentirely of inexpensive condensers and resistors, it being permissibleto even eliminate the transformers if impedance matching Vby other meansis provided. Because of the ease of balancing the circuits, Variationsin component tolerances over long periods of time can be easilycorrected without replacement of parts or further servicing. The entiretwo-way amplifier may be constructed in small light-weight units.

We claim:

1. A two-way amplifier system comprising two bridge networks, eachnetwork consisting of two impedance elements in adjacent arms and twoampliiier tubes in the other two arms of the bridge, a signal inputcircuit asymmetrically connected across one diagonal defined by thejunction points of each element and a tube, and a signal output circuitasymmetrically connected across the other diagonal Vand to the input ofone amplifier tube of the second network.

2. A two-way amplier comprising ayrst and a second Wheatstone bridge,each bridge having an amplifying tube in each of two adjacent arms andbalancing impedance elements in the remaining arms, a two-way signalcircuit coupled across the diagonal of each bridge defined by the apicesbetween the tubes and impedance elements, the other diagonal of eachbridge being coupled across the input of one tube of the oppositebridge.

3. In the two-way amplier defined in claim 2, each of said two-waysignal circuits positioned asymmetrically in the coupled diagonal withrespect to the electrical center of the diagonal.

4. In the two-way amplier defined in claim 2, each of the firstmentioned diagonale including the winding of a signal transformer and aseries impedance element, the junction of the winding and the serieselement being fixed at a grounding potential Vwith respect to theremainder of the bridge.

5. In the two-way amplifier dened in claim 2, including the mentionedcoupling between the input of one tube of each bridge and the diagonalof the opposite bridge, further comprising a coupling between the outputof said one tube of each bridge and the input of the tube of theadjacent arm of the bridge,

6. In. the two-way amplifier dened in claim 2, including the couplingbetween the input of said one tube and the opposite bridge, furthercomprising a common cathode resistor for the two tubes of each bridge,the input of the tube adjacent said one tube being coupled direct acrossthe common cathode resistor so that signals are amplied in cascade bythe tubes of each bridge and are impressed in phase opposition acrossthe dened diagonal of the bridge.

GEORGE E. BRODE. HAYES B. STEINHAUSER.

REFERENCES Ciani) The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,501,726 Rice July 15, 19242,085,488 Woodward et al. June 29, 1937 Disclaimer y 2,496,266.George E.Brode, Newark, and Hayes B. Stein/hanger, Mont-clair, N. J. Two-WAYAMPLIFIER FOR ELIMINATING OPPOSITELY DIRECTED SIGNALS. Patent dated Feb.7, 1950. Disclaimer filed Dec. 8, Y1951, by the assigne-e, FederalTelephone and Radio Corporation. i Hereby enters this disclaimer 'toclaims l to 4, inclusive, of said patent.

[Official Gazette January 15, 1959.]

